Concrete prestressing cable grip



July 24, 1962 A. R. ANDERSON 3,045,305

CONCRETE PRESTRESSING CABLE GRIP Filed Jan. 28, 1954 2 Sheets-Sheet 1INVENTO ARTHUR R. ANDERS July 24, 1962 Filed Jan. 28, 1954 A. R.ANDERSON CONCRETE PRESTRESSING CABLE GRIP 2 Sheets-Sheet 2 W 4 lI V 5455 INVENTOR.

ARTHUR A. ,4/v0ms0/v i: a: g

Z TTORNEYS United States 3,045,305 Patented July 24, 1962 ice 3,045,305CONCRETE PRESTRESSING CABLE GRW Arthur R. Anderson, Tacoma, Wash,assignor, by mesne assignments, to Concrete Technology Corporation,Tacoma, Wash, a corporation of Washington Filed Jan. 28, 1954, Ser. No.406,671

1 Claim. (Cl. 24-4216) The invention relates to apparatus'forprestressing concrete, and more particularly to an improved form of gripfor anchoring a group of wires under high tension in the casting of theprestressed concrete.

One of the most troublesome problems encountered in p the engineering ofprestressed concrete structures and their fabrication, has been toprovide a satisfactory way of anchoring the high-tensile steel wiresduring casting of the concrete. Under the extremely high tensions whichmust be imposed if the tensile strength of the wire is to be utilized tofull advantage, the wires frequently slip or break, whereas lowertensions can only have the result of lowering the strengthcharacteristics of the prestressed structures produced. Tests made withgrips constructed in accordance with this invention have demonstratedthat wires having a tensile strength of 265,000 p.s.i. (pounds persquare inch) can be anchored in groups of eight and pulled up to anaverage tension of as high as 250,000 p.s.i. before the first wirebreaks.

Summary According to my invention, there is provided a grip foranchoring a group of wires under high tension in the casting ofprestressed concrete, comprising a metal socket member having taperedinner Walls and a metal plug member received within the socket memberand having tapered walls complementary to the tapered inner walls of thesocket member, the tapered walls of one of the members havinglongitudinal grooves to receive the wires to be tensioned, the metal ofthe grooved member in the region of the grooves being of a hardnesssubstantially less than that of the surface of the wires so that as thewires are tensioned, portions of the surfaces of the wires are pressedinto the metal of the grooves, deforming the latter. The tapered wallsof the other .member are of a hardness at least equal to that of thesurface of the wires, so that the wires slide over such hard taperedwalls as the wires and grooved member are drawn into tight wedgingengagement with said other member.

Thus in one form of my invention, I provide a wire grip comprising afrusto-conical socket member and a complementary plug member receivedtherein; the plug having grooves to receive the Wires to be tensioned,this grooved member being made of aluminum, and the other member beingmade of hardened steel. Upon tensioning, the wires bite into thealuminum grooves as they slide over the hardened steel to wedge thewires tightly between the two members and lock them securely in thegrooves. What happens is that the wires die-form their own pattern intothe relatively soft aluminum, keying them to the aluminum member. Thisaction takes place as the hardened steel member moves relatively to boththe wires and the aluminum member, camming the wires into the aluminumto produce the aforesaid die-forming operation. Note that there issubstantially no deformation of the wire surfaces themselves andtherefore no substantial tendency to weaken the wires at the grip, whichis a common fault in certain types of grip used or proposed heretofore.

My invention is particularly advantageous as applied to the stressing ofstranded wire, i.e. wire made up of a group of smaller wires, or strandstwisted together as in a wire rope, as where a center wire, or core, issurrounded by, say, six other wires of the same size twisted helicallyaround the core Wire. In this case, when tension is applied thehelicalpattern of the strands is impressed into the initially smooth surface ofthe grooves in the member made of aluminum or other relatively softmetal. However, the invention is useful also in gripping solid wireshaving any surface deformations which will bite into the metal of thegrooves. Such deformations may be specially formed, or may compriseslight bends or irregularities existing in the wire as manufactured.Throughout the specification and claim, I shall employ the term wire inits generic sense as including both a single wire and a wire made up ofany number of single wire strands twisted together.

Description Referring to the drawings, I shall now describe the bestmode contemplated by me of carrying out my invention.

FIG. 1 is a general arrangement view illustrating my improved wire gripin use at one end of a prestressing concrete testing floor, and afragmentary plan view of the prestressing apparatus, showing one end ofthe mold (in horizontal section), the distributor plate at that end, andthe hydraulic jack for tensioning the wires anchored in the grip.

FIG. 2 is a central longitudinal sectional view of my preferred form ofgrip, taken as indicated at 22 in FIG. 4, with the wires in place,before tensioning.

FIG. 3 is a view similar to FIG. 2, after tensioning.

FIG. 4 is a transverse sectional view of the same grip, taken asindicated at 4-4 in FIG. 2.

FIG. 5 is an enlarged detail sectional view taken in the same plane asFIG. 4, but illustrating the condition which exists after tensioning.

FIG. 6 is an elevational view of the plug member of the same grip, as itappears before use.

FIG. 7 is a similar view of the plug, as it appears after use.

FIG. 8 is a central longitudinal sectional view of a plug of modifiedconstruction.

FIG. 9 is a sectional view similar to FIG. 2, illustrating a furthermodification taken as shown at 9--9 in FIG. 10.

FIG. 10 is a transverse sectional view, taken as shown at 10-10 in FIG.9.

FIG. 11 is a detail perspective view of a solid wire of a form adaptedfor anchoring by the use of my improved grip.

In FIG. 1, I have shown one end of a mold 12 for casting a prestressedconcrete beam. A group of high tensile steel wires 13 extends throughthe mold lengthwise of the beam, the position of the respective wires inthe mold being determined by distributor plates 14 at the ends of themold, these plates being perforated to receive the wires. From thedistributor plate at one end of the mold, the group of wires 13 aregathered into the grip 15 by which they are to be anchored. Grip 15bears against a cross-head plate 16 in which there is an opening topermit the group of wires to pass through. For prestressing the wires,hydraulic jacks, one of which is shown at 17, act against a heavyreinforced concrete abutment 18, to apply tension through cross-head 19and rods 20 connecting cross-head 19 with cross-head 16. Rods 20 extendthrough holes in the concrete abutment 18.

It will be understood that similar anchoring means are provided at theother end of the mold (not shown) including another wire grip 15 and across-head or other device for holding it in place during theapplication and maintenance of tension by the hydraulic jack 17,described. It'will also be understood that, depending upon the natureand design of the prestressed structure being manufactured, there may beseveral groups of wires 13 arranged, for example, one above the other inengagement with cross-head plate 16, or attached to separate tensioningmeans, as may be desired.

Referring to FIGS. 2 and 4, the grip comprises a metal socket 21 havingtapered inner wall portions 22 of frusto-conical form, and a metal plug23 received within socket 21 and having tapered wall portions 24 (FIG.6) complementary to the tapered inner wall portions of the socket, thetapered wall portions of the plug having longitudinal grooves to receivethe wires 13 to be tensioned. The metal of the plug 23 in the region ofgrooves 25 is of a hardness substantially less than that of the surfaceof the wires 13 so that as the wires are tensioned, portions of thesurfaces of the wires are pressed into the metal of the plug, deformingthe latter. Further, the tapered inner wall portions 22 of the sockethas a hardness at least equal to that of the surface of the wires, sothat when tension is applied, the hard inner wall portions 22 of thesocket slides over the wires without substantial deformation of thesocket, as portions of the surfaces of the wires are pressed into themetal of the plug, deforming the latter while the socket is drawn intotight wcdging engagement with the wires and plug. Thus, as the socket 21slides relatively to the wires 13, the pattern of the strands of thewires is impressed into the metal of plug 23 to lock the plug and wiresagainst relative longitudinal movement. Movement of the socket 21relatively to the wires 13 and plug 23 is indicated schematically inFIGS. 2 and 3. FIG. 2 shows the relative positions of the members beforetensioning, and FIG. 3 the relative positions after tensioning. Theextent of relative movement is indicated by comparing these two viewsand would be equal to a minus b. However, the drawings have not beenprepared to scale, and therefore are not intended to indicate the extentof this movement quantitatively. In practice, this will vary and willdepend to some extent upon the form of wires 13 and the difference inrelative hardness between the wires and the plug.

FIGS. 2 and 4 represent the condition before tension has been applied tothe wires, when the grooves 25 are smooth. FIGS. 3 and 5 represent thecondition after tensioning, when the pattern of the strands of the wireshas been impressed into the metal of the plug to lock the plug and wiresagainst relative longitudinal movement.

The effect of the action described will further be understood by acomparison of FIGS. 6 and 7, FIG. 6 showing the plug before use, andFIG. 7 the same plug after use. Note that in FIG. 6 the grooves 25 aresmooth whereas in FIG. 7 they are impressed with the pattern of thestrands of wires 13.

In practice LI have generally made the socket 21 of steel, casehardened, and the plug 23 of an aluminum alloy having a Brinell hardnessof not substantially more than about 70 (50.0 kg., 10 mm. ball), andpreferably having a Brinell hardness of between about 45 and 70. Thetapered inner walls 22 of the socket may be case hardened to a Brinellhardness of between about 650 and 700 (3000 kg., 10 mm. ball). The testswhich I have performed reveal that good results are obtainable withplugs made of zinc and of copper, as well as of aluminum. The essentialpoint appears to be the use of a material which is deformable by thewires to be tensioned under normal wedging conditions produced by thetensioning itself. 7

The metal of the plug in the region of the grooves should be relativelysoft as compared with the hardness of the tapered inner wall portions ofthe socket. The

' wires, this being largely a matter of choice.

4 plug need not be formed entirely of the relatively soft metal, butcould if desired have a hard core such as the steel core 26 of themodified plug construction shown in FIG. 8.

In FIGS. 9 and 10 I have illustrated a modified form of grip in whichthe socket 27 and plug 28 are of rectangular form instead of round. Asin the embodiment previously described, the socket has tapered innerwall portions 29 and the plug has tapered inner wall portions 30complementary to the tapered inner wall portions of the socket, the plugalso having longitudinal grooves 31 to receive the wires 13 to betensioned. The same relative hardness conditions are observed here as inthe previous embodiment. In all cases I have found 1 it helpful to havethe socket entrance flared and rounded (32, FIG. 2; 33, FIG. 9). This isa point of high stress concentration, and a carefully contouredcurvature in the socket entrance is important to relieve the stressconcentration.

The plugs may be grooved to accommodate two, three, four, six or eightWires, solid or stranded. In producing large beams, I have usedthirty-two wires with four grips at each end of the mold, eight wiresper grip. If desired, the grips could be designed to carry a greaternumber of While I have illustrated in the drawing the use of multi-wirestrands (the strands shown having seven wires), my grip is useful alsoin anchoring solid wires having surface deformations which will biteinto the metal of the grooves. Such deformations may be speciallyformed, such as the lugs 34 of wire 35 shown in FIG. 11, or may compriseslight bends or irregularities existing in the wires manufactured.

The terms and expressions which I have employed are used in adescriptive and not a limiting sense, and I have no intention ofexcluding such equivalents of the invention described, or of portionsthereof, as fall within the purview of the claim.

I claim:

A grip for anchoring a group of wires under tension in stressed concretework without requiring the use of auxiliary clamping devices orprocedures to effect clamping, comprising a metal socket having taperedinner wall portions and a metal plug received within said socket havingtapered wall portions complementary to the tapered inner wall portionsof the socket, said tapered wall portions of the plug havinglongitudinal grooves to receive the wires to be tensioned, the metal ofsaid plug in the region of said grooves being of a hardnesssubstantially less than that of the surface of the wires and the taperedinner wall portions of the socket being smooth and free of grooving atpoints opposite the longitudinal grooves of the plug and being of ahardness at least equal to that of the surface of the wires.

References Cited in the file of this patent UNITED STATES PATENTS1,297,187 Lamb Mar. 11, 1919 1,642,628 Philbrick Sept. 13, 19271,643,110 Briggs Sept. 20, 1927 1,706,805 Moffitt Mar. 26, 19291,758,312 De Right May 13, 1930 1,886,247 Cole Nov. 1, 1932 2,017,887Blackburn Oct. 22, 1935 2,042,090 Cummins May 26, 1936 2,294,398Ferguson Sept. 1, 1942 2,341,922 King Feb. 15, 1944 2,686,963 FreyssinetAug. 24, 1954

